On Thu, May 30, 2024, Igor Mammedov wrote: > On Thu, 30 May 2024 21:54:47 +0800 Zhao Liu <zhao1....@intel.com> wrote:
... > > > >> following, CPUID_EXT_MONITOR is set after x86_cpu_filter_features(), so > > > >> that it doesn't have a chance to check MWAIT against host features and > > > >> will be advertised to the guest regardless of whether it's supported by > > > >> the host or not. > > > >> > > > >> x86_cpu_realizefn() > > > >> x86_cpu_filter_features() > > > >> cpu_exec_realizefn() > > > >> kvm_cpu_realizefn > > > >> host_cpu_realizefn > > > >> host_cpu_enable_cpu_pm > > > >> env->features[FEAT_1_ECX] |= CPUID_EXT_MONITOR; > > > >> > > > >> > > > >> If it's not supported by the host, executing MONITOR or MWAIT > > > >> instructions from the guest triggers #UD, no matter MWAIT_EXITING > > > >> control is set or not. > > > > > > > > If I recall right, kvm was able to emulate mwait/monitor. > > > > So question is why it leads to exception instead? Because KVM doesn't emulated MONITOR/MWAIT on #UD. > > > KVM can come to play only iff it can trigger MWAIT/MONITOR VM exits. I > > > didn't find explicit proof from Intel SDM that #UD exceptions take > > > precedence over MWAIT/MONITOR VM exits, but this is my speculation. Yeah, typically #UD takes priority over VM-Exit interception checks. AMD's APM is much more explicit and states that all exceptions are checked on MONITOR/MWAIT before the interception check. > > > For example, in ancient machines which don't support MWAIT yet, the only > > > way it can do is #UD, not MWAIT VM exit? Not really relevant, because such CPUs wouldn't have MWAIT-exiting. > > For the Host which doesn't support MWAIT, it shouldn't have the VMX > > control bit for mwait exit either, right? > > > > Could you pls check this on your machine? If VMX doesn't support this > > exit event, then triggering an exception will make sense. > > My assumption (probably wrong) was that KVM would emulate mwait if it's > unavailable, Nope. In order to limit the attack surface of the emulator on modern CPUs, KVM only emulates select instructions in response to a #UD. But even if KVM did emulate MONITOR/MWAIT on #UD, this is inarguably a QEMU bug, e.g. QEMU will effectively coerce the guest into using a idle-polling mechanism.
